Portable heat storage units have gained wide acceptance for therapeutic purposes and for a wide variety of applications that call for heat on demand. For this use phase change materials which give off heat in changing from supercooled liquid form to crystalline state have been found particularly useful since they give off heat at a predetermined and constant temperature during the time of application which may be several hours.
Reliable heat storage requires that the supercooled condition be stable for extended periods; but this stability in turn creates difficulties in initiating crystallization. For example, sodium thiosulfate pentahydrate, hereafter referred to as STP, has a uniquely high tendency to remain in supercooled liquid form for practically indefinite time at room temperature and has a large heat of fusion in latent form. Because of the unusually large activation energy necessary for nucleation and because of the low symmetry of the crystals, it has, up to the present, required crystals of the substance itself as seed for epitactic nucleation and growth.
Nucleation of such materials can also be induced by surface energy in the form of dislocations and surface charge on a variety of materials when they are in active state. Thus, STP has been nucleated by this procedure by sodium tetraborate decahydrate and sodium sulfite heptahydrate. However, if such substances, as is necessary in the case of recyclable systems, are heated together with the phase change material, such agents consistently lose their nucleating power and do not regain their active state unless they are exposed to the atmosphere and in some cases reactivated by grinding or other means.